At present, various three-level circuits are widely used in many fields due to their advantage of high efficiency. A common Vienna-like three-level circuit is shown in FIG. 1, which includes switch transistors Q1 and Q2 which constitute a power-frequency bridge arm, switch transistors S1 and S3 which constitute a first high-frequency bridge arm, switch transistors S2 and S4 which constitute a second high-frequency bridge arm, inductors L1 and L2, and bus capacitors Co+ and Co−, where the first high-frequency bridge arm and the second high-frequency bridge arm are connected in series to form a branch, where two terminals of the branch are DC output terminals of the Vienna-like three-level circuit; a connection node between the first high-frequency bridge arm and the second high-frequency bridge arm and a middle point of the power-frequency bridge arm are AC input terminals of the Vienna-like three-level circuit; one terminal of the power-frequency bridge arm is connected to a middle point of the first high-frequency bridge arm via the inductor L1, and the other terminal of the power-frequency bridge arm is connected to a middle point of the second high-frequency bridge arm via the inductor L2; and the bus capacitor Co+ is connected in parallel between two terminals of the first high-frequency bridge arm, and the bus capacitor Co− is connected in parallel between two terminals of the second high-frequency bridge arm. Specifically, The switch transistors Q1 and Q2 may be uncontrolled transistors, half-controlled thyristors, or fully-controlled MOS transistors or IGBTs, switch transistors S1, S2, S2 and S4 are fully-controlled MOS transistors or IGBTs, diodes D1, D2, D3 and D4 are body diodes or external anti-parallel connected diodes of the switch transistors S1, S2, S3 and S4 respectively, and capacitors Coss1, Coss2, Coss3 and Coss4 are equivalent junction capacitors of the switch transistors S1, S2, S3 and S4 respectively.
In the Vienna-like three-level circuit shown in FIG. 1, during a positive half cycle of an AC input voltage Vin, a switch transistor Q1 is in an on state, a first high-frequency bridge arm is in an operation state, a switch transistor S3 is the main switch transistor, and a switch transistor S1 is the freewheeling switch transistor; during a negative half cycle of the AC input voltage Vin, a switch transistor Q2 is in an on state, a second high-frequency bridge arm is in an operation state, a switch transistor S2 is the main switch transistor, and a switch transistor S4 is the freewheeling switch transistor.
In the conventional technology, a commonly used method for controlling the Vienna-like three-level circuit is an inductor current continuous conduction mode control method such as the CRM (Critical Conduction Mode) control method. However, a zero-voltage turning on of the high-frequency bridge arm switch transistor cannot be achieved with this control method, thereby resulting in high circuit loss.